Enucleate condition is the correct answer for the common structural feature of vessel elements and sieve tube elements. Both vessel elements and sieve tube elements are types of cells found in the conducting tissues of plants. These cells are responsible for the transport of water, nutrients, and sugars throughout the plant.

Here is a detailed explanation of why enucleate condition is the correct answer:

1. Vessel elements:
Vessel elements are part of the xylem tissue in plants and are involved in the transport of water and minerals from the roots to the leaves. They are long, cylindrical cells that are stacked end to end to form long tubes called vessels. Vessel elements have the following structural features:
- Pores on lateral walls: Vessel elements have perforations or pores on their lateral walls. These pores allow for the movement of water and dissolved minerals between adjacent vessel elements, creating a continuous pathway for water transport.
- Thick secondary walls: Vessel elements have thick secondary walls composed of a substance called lignin. Lignin provides strength and support to the cells, allowing them to withstand the pressure generated by the upward movement of water.

2. Sieve tube elements:
Sieve tube elements are part of the phloem tissue in plants and are involved in the transport of sugars and other organic compounds from the leaves to other parts of the plant. They are elongated cells that are arranged end to end to form sieve tubes. Sieve tube elements have the following structural features:
- P-protein: Sieve tube elements contain a specialized protein called P-protein. P-protein forms dense granules or bodies called P-bodies that are located near the sieve plates, which are sieve-like structures between adjacent sieve tube elements. The P-protein helps in sealing damaged sieve tubes to prevent the loss of sap.
- Enucleate condition: One of the unique features of sieve tube elements is that they lack a nucleus at maturity. The nucleus and other organelles are gradually broken down and eliminated during development, resulting in enucleate sieve tube elements. This adaptation allows for a more efficient flow of sap through the sieve tubes.

In summary, the common structural feature of vessel elements and sieve tube elements is the enucleate condition, where sieve tube elements lack a nucleus at maturity. This feature allows for a more streamlined flow of sap in the sieve tubes, while vessel elements have pores on their lateral walls and thick secondary walls.